This post is part of a series: Building the VK3YE Beach 40 DSB Transceiver.
After listening some of of the recent episodes of the Soldersmoke podcast, and hearing about their adventures great and small having to do with VU2ESE’s BITX SSB transceiver, I’ve got a hankering to build something from scratch. Rather than jump in at the deep end, as it were, with an SSB transceiver, I’m starting with something more approachable: Peter Parker VK3YE’s “Beach 40” Double Sideband transceiver.
There’s a host of information on the internet about this project, but Peter’s project page, linked above, is a good place to start. You can also find some expanded notes on the build in a write-up of the Beach 40 Lo-Key magazine at the time, several years back.
The project is conceptually simple: A balanced diode ring mixer functions as the product detector on receive, and as the balanced modulator on transmit. Hook up the RF-pre-amp and an LM-386 audio amp, it’s a receiver. Hook up a mic amplifier and an RF power chain, it’s a transmitter. No muss no fuss. A handful of transistors, one IC, about 2-3W RF output.
Peter’s original design used a ceramic resonator in the VXO, though later versions of his employed a “super-VXO,” in which two or more similar crystals are paralleled for extra pulling range. I decided to use a true LC VFO in mine, as I’d like to cover from 7.125 to 7.300, the entire USA SSB range, if I can.
This is VFO design I started with. It’s a Hartley oscillator straight out of EMRFD. To that, I added a simple common collector buffer stage:
Unfortunately, this design didn’t produce a high enough power level to drive a diode-ring mixer. Traditional diode ring mixers want to be driver with around +7dBm available power on their LO port, and I was only seeing 2 dBm. So I added an additional common emitter power stage to boost the signal level.
Once the thing was oscillating at the proper power level, I spent a good hour tweaking the inductors and capacitors in the oscillator’s tank circuit to get the frequency spread down to the appropriate range in the 40m band. The final result is actually a bit wider than I’d like it to be, so some additional tweaking in the future will be necessary. I also found that, as I added additional shunt capacitance to the tank, I had to increase the coupling capacitor (originally 47pF, now 220pF), to get the circuit to continue to oscillate reliably. Here’s what I finally ended up with:
One thing this process really drove home is the importance of shielding the VFO – the hand capacitance effects alone during debugging were making me a bit crazy. Sometimes just reaching in to hook up a scope probe would cause the thing to stop oscillating or start oscillating, or develop some parasitic oscillation…. woof. I’ll have to build a little box for the thing now that it’s essentially in the right shape. But for the moment, here it is, in all is little glory:
As I’ve documented before, I find that a variation on the Island Squares method of circuit construction is very approachable for this kind of circuit building. Essentially, small isolated pads are cut into a piece of copper-clad board using a dremel, and the components soldered to those. Ground connections attach to the ground plane of the board. It’s easy and quick, and allows for long connections to be made when necessary (the 12V bus on this project spans one whole side of the board, with de-coupling along the way).
I’m planning to build the next stages in the order Peter suggests in his Lo-Key article: Audio output amplifier, the diode-ring mixer, low-pass filter, mic amplifier, and the RF power chain. Since he originally published the design, Peter’s changed from a discrete-component to an LM386 audio amp, added an RF pre-amp, and stuck an L-Match tuner inside the case as well. I’m planning on adding the first two modifications, and perhaps an outboard unit for the third.
Hear you on the Air!